Since we can have multiple sites tracked by one piwik installation it would be good to have a separate opt-out for each site.
Turning tracking off on one site shall not switch tracking of another site tracked by the same piwik instance. It might confuse users coming to a site they never were and finding tracking turned off "by default". Just in case they turn it on, i.e. to get better recommendations on a specific site, and tracking switches automatically on another site as well even having turned it off before might cause troubles for the site owners.

Andreas